1. Field of the Invention
The present invention relates generally to the field of nuclear reactor primary system decontamination and, more particularly, to a scheme for locating components forming a clean-up system utilized to remove suspended and dissolved solids from nuclear reactor primary system fluids after the primary system has been subjected to a chemical decontamination process.
2. Description of the Prior Art
The problem of excessive personnel exposures caused by high background radiation levels in a nuclear reactor primary system, such as in a pressurized water reactor (PWR) system, and the resultant economic cost of requiring personnel rotation to minimize individual exposure is significant at many nuclear plants. These background levels are principally due to the build-up of corrosion products in certain areas of the plant. The build-up of corrosion products exposes workers to high radiation levels during routine maintenance and refueling outages.
As a nuclear power plant operates, the surfaces in the core and other portions of the primary system corrode. Corrosion products, referred to as "crud", are activated by transport of the corroded material to the core region by the reactor coolant system. Subsequent release of the activated crud and redeposition elsewhere in the system produces radiation fields in piping and components throughout the primary system, thus increasing radiation levels throughout the plant. The activity of the corrosion product deposits is predominantly due to cobalt 58 and cobalt 60. It is estimated that between 80% and 90% of personnel radiation exposure can be attributed to these elements.
One way of controlling worker exposure, and of dealing with this problematic situation, is to periodically decontaminate the nuclear steam supply system using chemicals, thereby removing a significant fraction of the corrosion product oxide films. Early techniques did very little to decontaminate the primary system as a whole, typically focusing only on the heat exchanger (steam generator) channel heads.
Two different chemical processes, referred to as LOMI (developed in England under a joint program by EPRI and the Central Electricity Generating Board) and CAN-DEREM (developed by Atomic Energy of Canada, Ltd.), were initially suggested for small scale decontamination. These processes are multi-step operations, in which various chemicals are injected, recirculated, and then removed by ion exchange. Although the chemicals are designed to dissolve the corrosion products, some particulates are also generated. Another method of chemical decontamination which focuses on the chemistry of decontamination is disclosed in U.K. Patent Application No. GB 2 085 215 A (Bradbury et al.).
While these chemical processes were initially used only on a localized basis, the use of these chemical processes has now been considered for application on a large scale, full system chemical decontamination. Such an application is disclosed generally in U.S. Pat. No. 5,089,216, assigned to the assignee of the present invention and entitled "System for Chemical Decontamination of Nuclear Reactor Primary Systems".
The estimated collective radiation dose savings over a 10-year period following nuclear reactor primary system decontamination is on the order of 2,500-4,500 man rem, depending upon whether or not the fuel is removed during decontamination. At any reasonable assigning of cost per man rem, the savings resulting from reduced dosage levels will be in the tens of millions of dollars.
As a result of the recognition that full nuclear reactor primary system chemical decontamination is indeed possible from a technical standpoint, and more importantly, advantageous in order to increase collective radiation dose savings, systems have been suggested for effective and economic removal of dissolved and particulated corrosion products generated by the application of these known chemical decontamination techniques. One such system is disclosed in U.S. Pat. No. 5,089,217, assigned to the assignee of the present invention and entitled "Clean-up Sub-System for Chemical Decontamination of Nuclear Reactor Primary Systems". In this sub-system, chemically processed fluids containing suspended and dissolved solids are directed first through a back-flushable filter and, thereafter, through one or more cartridge filters. After this initial filtering of suspended solids, these fluids are directed to one or more banks of demineralizers for removal of dissolved solids, followed by additional cartridge filtering to remove any resin fines carried out of the demineralizer banks. After final filtering, the fluids are returned to the primary system. The clean-up sub-system described in this patent requires that the resin beds within each of the demineralizers be replaced at least once during the chemical decontamination clean-up process. U.S. patent application Ser. No. 07/630,049, filed Dec. 19, 1990, now U.S. Pat. No. 5,171,519, entitled "Outside of Containment Chemical Decontamination System for Nuclear Reactor Primary Systems" and assigned to the assignee of the present invention, teaches locating the components of the chemical decontamination clean-up system outside of containment in a specially constructed, shielded building.
U.S. patent application Ser. No. 07/983,503, filed Nov. 30, 1992, now U.S. Pat. No. 5,325,410, entitled "Clean-up System for the Chemical Decontamination of a Nuclear Reactor Primary System", assigned to the assignee of the present invention and incorporated herein by reference, discloses a clean-up system which improves on prior clean-up systems and effectively and economically removes dissolved and particulated corrosion products from primary system fluids generated by the application of known chemical decontamination techniques. This clean-up system does not require demineralizer resin bed replacement during the decontamination clean-up process. While the equipment forming the improved clean-up system disclosed in U.S. patent application Ser. No. 07/983,503, filed Nov. 30, 1992, now U.S. Pat. No. 5,325,410 may be housed in its own specially constructed decontamination clean-up building as disclosed in patent application Ser. No. 07/630,049 filed Dec. 19, 1990, now U.S. Pat. No. 5,171,519 if building space is readily available, constructing a separate decontamination building complete with necessary shielding, curb retainers and containment drains may not always be preferred due to limited available space and cost limitations.
Therefore, there exists a need for a chemical decontamination clean-up system design layout which utilizes existing available space to house components of the clean-up system and thereby avoids the costs and other logistics difficulties associated with the construction of a separate clean-up building. The design would be an "outside of containment" design in that the various components which constitute the chemical decontamination clean-up process would be installed outside of the nuclear reactor containment chamber.